Unless you want to bump up to i7, the i5's are better than the Quads. Mhz can only really be used to compare similar processors these days as there's a lot more to them.
Right, so I've done this one:
http://www.thepcogroup.co.uk/compute...custombuilt=93
*Bumped the motherboard up to the USB 3.0 one (for an extra £20, may as well future-proof it a tad)
*Bumped the RAM up to 4gb 1600Mhz. I'd reccomend 4gb and the extra £10 spent on faster RAM is worth it. Remember though, you'll need a 64-bit OS to fully take advantage of it.
*GFX - Go for the 1gb HD5570 or higher, depending on your budget. Again, the extra £10 or so difference between the 512mb and 1gb models will make a difference. If you can afford a 5850 then I would.
That makes it £524 (with the 1gb ATI HD5570) without fiddling with hard drives, DVD Drives and software, but you'll know what you want in that respect